ESD structures serve to protect electronic circuit blocks against electrostatic discharge pulses or further current pulses which can occur, for instance, during the production of the integrated circuit or during the operation thereof. Examples of such pulses are HBM (HBM: Human Body Model) pulses in accordance with DIN IEC 60749-26, MM (MM: Machine Model) pulses in accordance with DIN IEC 60749-27, CDM (CDM: Charged Device Model) pulses in accordance with DIN IEC 60749-28. Without ESD structures there is the threat of destruction of useful components of the circuit blocks to be protected, e.g. by current or voltage overloading, which can lead for example to short circuits, an increase in leakage currents or else to defective gate oxides. This can lead to the loss of the functionality of the integrated circuit.
ESD structures typically comprise protection elements such as e.g. NPN or PNP bipolar transistors designed therefor, parasitic NPN or PNP bipolar transistors, SCRs (Silicon Controlled Rectifiers) or thyristors. The expression parasitic bipolar transistor is used herein for a sequence of NPN or PNP regions of arbitrary useful components whose NPN or PNP regions can act as a bipolar transistor.
ESD structures typically have breakdown voltages which lie above the maximum operating voltage of the pin to be protected and below the destruction voltage of the useful components connected to said pin. Typically, in the range of these two voltages in the case of an electrostatic discharge process the ESD structure begins to carry away discharge current. In this case, it is desirable for the breakdown voltage of the ESD structure to have a smallest possible separating interval from the maximum operating voltage at the pin to be protected, in order to have available a largest possible voltage window up to the destruction voltage of the useful components for the operation of the ESD structure. Limits are imposed on the minimization of said separating interval for instance by process fluctuations of the breakdown voltage and the temperature dependence of the breakdown voltage.